Conventional technologies of narrow band filters are typically achieved with thin film filters that are designed to have one pass-band with flat pass-band and sharp stop-band as required in telecom applications. With evolving technology and complexity of telecommunications needs, these confronted with technical difficulties that applications of the thin film filtering technologies are no longer practical due to the requirements multiple wavelength separation and selections. The requirement is demanded by the recent developments of fiber optical telecommunication systems that employ multiplexed optical signals for signal transmission, thus requires separation of signals carried in multiple wavelength ranges. In order to achieve the tasks of separating multiple wavelengths, cascaded thin film filters each performs the function of narrow band filtering are required. For applications that involve separation of multiple wavelength signal transmissions, grating based technologies are generally applied. However, compared to the thin film filters, the grating based devices, despite a lower per channel cost, are usually implemented at a higher overall cost while providing lower optical performance characteristics such as rounded filter pass-band and slewed stop-band that results in low channel isolation.
Another difficulty of the convention narrow band filters when implemented in telecommunication system is the implementation of filters of fixed center wavelengths. Due to the fact that the center wavelengths of these filters are fixed, once a telecommunication system is completed, it is extremely difficult to reconfigure or re-route network traffic. As the high-speed network expands, so does the complication of network management and there are increased demands to reconfigure and re-route the signal transmissions. However, the demands for reconfiguration and re-routing generally requires the implementation of tunable optical devices including tunable lasers, switches and optical add/drop modules that can be conveniently reconfigured. When there are continuous expansions of the optical fiber system, these tunable optical devices, including tunable filters become the focus of attention in the development of next wave of networking technology advancement. Tunable filters start to become available among active components vendors. Available ones include MEMS and grating based filters that involve mechanical moving parts, liquid crystal ones that are electro-absorptive, thin film ones that are thermally tuned. However, these filters either lack the optical performance characteristics as demanded by the modern more complicate systems for telecommunications or lack of mechanical stability/aging/repeatability related performance characteristics. The tunable filters as now available also are often useful only for single channel application and expensive to produce.
For these reasons, there is still a need in the art to provide an improved design and manufacture process to provide a narrow band tunable filter to overcome the above-mentioned difficulties. It is desirable that the new and improved tunable filter can be produced with high level of optical performance characteristics and low cost.